You're literally in a metagame thread farthest from what you're searching for, this metagame centralizes on using mega stones with almost any pokemon, there is nothing about move replacing or learning, this is not hackmons. And since Rayquaza has no stone, his mega can't be used. It probably works with Smeargle if the code allows it, but I 100% doubt it.
Since Smeargle has been banned from Mega Evolution... no, it's not legal on anything currently.
(whoops, forgot to cover Delta Stream, though it doesn't matter, so whatever)
As grateful as you've been explaining the near-uselessness of Trace, I don't see how running trace and hyper voice on the same mon is useless when Zygarde is S-rank, and popular with Altarianite. Grass Knot is also boosted by Tough Claws, though there's no good alakazite user with Grass Knot anyway so that point is moot.
It's an option for Togekiss, the mon itself has boosting moves in the form of tailwind and nasty plot, as well as a decent move pool to abuse nasty plot. Also the 85/115/115 is a nice mixed bulk on top of a typing that, again, abuses both Aerilate and Pixilate equally. I'd like to point out its utility when it picks up these certain traceable moves. Also, depending on speeds and mons, you can trace some abilities pre-mega, like Prankster, Natural Cure, Regenerator.
If you want a Special attacker with Pixilated Hyper Voice (Boomburst, Wring Out, whatever) to deal with Zygarde, you're better off with Altarianite or Gardevoirite -other than being 10 less Speed Gardevoirite provides the same statline. (Plus 20 pointless Attack) Trace is a bad gimmicky way of accomplishing that, especially since you'll outspeed Zygarde or fail to outspeed Zygarde (Because Dragon Dance) regardless, with Togekiss (Among other possibilities), whether you're Alakazite or Gardevoirite.
Anytime you don't Trace an -ate Ability, Hyper Voice is functionally a dead slot, which is terrible, and not something you can afford. Aside from Pixilate Zygarde, few Pokemon hate being hit with their -ate type: Altarianite Entei and Arcanine are resistant, Pinsirite Zygarde is neutral, Glalitite Weavile is resistant...
Meanwhile, as far as Tracing pre-Mega, you can look to Trace's utility in OU. (And then lower your estimation of its utility because you'll get any given use once, at most) I personally
love Mega Gardevoir in way too many tiers, so I have a lot of experience with Trace, and it's utility goes like this:
-If Shedinja can be successfully Traced, that can be a sweep right there. (Nobody runs Shedinja in MnM, and nobody will until a Sturdy Mega Stone is released... and if that ever happens it will probably result in a ban, anyway)
-Intimidate is a great Trace
because Intimidators are almost all Physical. (Mega Manectric is the only viable exception in Standard) In MnM, that will only apply the first time, unless it's Manectite, in which case they aren't Physical, and it's useless. In practice this amounts to an opportunity to Intimidate Landorus-Therian one time... and you could run Manectite for a similar statline and
reliable, repeatable Intimidate.
-Flashfire is an excellent Trace because almost everything that runs it defaults to Fire as an attack. (Heatran is literally the only viable, Flashfire-running MnM Pokemon, and it will only work the one time) This extends, to a smaller extent, to the Water and Electric immunity Abilities. (With the caveat that almost nothing in MnM that gets them is run at all, or at least run with them) Furthermore, all three of these typings have better ways to become immune to them in MnM: Red Orb for Water, Blue Orb for Fire, Sceptilite for Electric.
-Unaware is a decent Trace, specifically if Clefable, because Clefable often runs either Calm Mind or Cosmic Power, and being able to ignore that is valuable. (Nobody runs Unaware in MnM)
-Regenerator is a decent Trace... primarily because Gardevoir has crap recovery, and will often want to switch out after forcing a switch anyway. (Nobody runs Regenerator in MnM, even though it could theoretically be useful)
-Natural Cure is an OK Trace, entirely because Starmie and Chansey carry Thunder Wave and it sucks to be hit by it. (Well, and some extremely passive Chansey rely entirely on Toxic) This all runs into the problem, in MnM, that Absolite is vastly superior protection, plus the secondary problem that you'll only get to do it the one time,
again, since they'll have Mega Evolved immediately.
-Tracing Sand Rush, Swift Swim, or Chlorophyll can be excellent for counter-Speeding the abuser, when the situation crops up. (Weather teams are not a thing in MnM, and are limited in their potential to be a thing, barring further weather-oriented Mega Evolutions being released)
-Tracing Shadow Tag has the utility of allowing you to switch out of it. (Banned from MnM in its entirety)
-Download is occasionally useful if you get the correct boost. (Downloaders are not viable in MnM, at least not enough to be worth commentary, and it's a one-time deal, again)
-Forewarning you that you're facing Zoroark by virtue of it failing to trigger can be useful. (Nobody runs Zoroark in MnM)
-Multiscale. It's Multiscale. (Could have a niche use against Lugia, I'll admit)
-Poison Heal. It's not unusual for Gliscor to try to stall out Pokemon with Toxic+Poison Heal. (Nobody runs Poison Heal in MnM)
This is about it, barring
extremely niche things like switching into a Sturdy Steel type that would otherwise OHKO you. There's things you might
expect to be useful -shouldn't Magic Guard be a good Trace?- but the meta doesn't accommodate that expectation. (Magic Guarders rarely employ indirect damage themselves)
The only Tracer I've ever used in any metagame is Gardevoir, and entirely because Telepathy is useless in Singles while Synchronize is
even more niche and unreliable than Trace. Trace itself is a limited Ability, in Standard, where it's more viable than in MnM.
To really argue for Trace/Alakazite, your Pokemon needs to simultaneously fulfill multiple roles against common threats entirely on the basis of Trace, such as acting as a check on a slower Banettite user
and walling Sablenite users who like to dump hazards
and... something else, I dunno...
And thus far nobody has found such a thing or even theorymonned it in any capacity. The Sceptilite Electrify Heliolisk counter is about it, and frankly the only reason I credit it at all is because it means you can run Sceptilite Heliolisk alongside your Sceptilite Heliolisk counter. Otherwise I'd just say "so why not run Sceptilite on Togekiss?"